fsfw/globalfunctions/DleEncoder.cpp
2020-08-26 19:59:58 +02:00

125 lines
3.4 KiB
C++

#include "../globalfunctions/DleEncoder.h"
DleEncoder::DleEncoder() {}
DleEncoder::~DleEncoder() {}
ReturnValue_t DleEncoder::encode(const uint8_t* sourceStream,
size_t sourceLen, uint8_t* destStream, size_t maxDestLen,
size_t* encodedLen, bool addStxEtx) {
if (maxDestLen < 2) {
return STREAM_TOO_SHORT;
}
size_t encodedIndex = 0, sourceIndex = 0;
uint8_t nextByte;
if (addStxEtx) {
destStream[0] = STX_CHAR;
++encodedIndex;
}
while (encodedIndex < maxDestLen and sourceIndex < sourceLen)
{
nextByte = sourceStream[sourceIndex];
// STX, ETX and CR characters in the stream need to be escaped with DLE
if (nextByte == STX_CHAR or nextByte == ETX_CHAR or nextByte == CARRIAGE_RETURN) {
if (encodedIndex + 1 >= maxDestLen) {
return STREAM_TOO_SHORT;
}
else {
destStream[encodedIndex] = DLE_CHAR;
++encodedIndex;
/* Escaped byte will be actual byte + 0x40. This prevents
* STX, ETX, and carriage return characters from appearing
* in the encoded data stream at all, so when polling an
* encoded stream, the transmission can be stopped at ETX.
* 0x40 was chosen at random with special requirements:
* - Prevent going from one control char to another
* - Prevent overflow for common characters */
destStream[encodedIndex] = nextByte + 0x40;
}
}
// DLE characters are simply escaped with DLE.
else if (nextByte == DLE_CHAR) {
if (encodedIndex + 1 >= maxDestLen) {
return STREAM_TOO_SHORT;
}
else {
destStream[encodedIndex] = DLE_CHAR;
++encodedIndex;
destStream[encodedIndex] = DLE_CHAR;
}
}
else {
destStream[encodedIndex] = nextByte;
}
++encodedIndex;
++sourceIndex;
}
if (sourceIndex == sourceLen and encodedIndex < maxDestLen) {
if (addStxEtx) {
destStream[encodedIndex] = ETX_CHAR;
++encodedIndex;
}
*encodedLen = encodedIndex;
return RETURN_OK;
}
else {
return STREAM_TOO_SHORT;
}
}
ReturnValue_t DleEncoder::decode(const uint8_t *sourceStream,
size_t sourceStreamLen, size_t *readLen, uint8_t *destStream,
size_t maxDestStreamlen, size_t *decodedLen) {
size_t encodedIndex = 0, decodedIndex = 0;
uint8_t nextByte;
if (*sourceStream != STX_CHAR) {
return DECODING_ERROR;
}
++encodedIndex;
while ((encodedIndex < sourceStreamLen) && (decodedIndex < maxDestStreamlen)
&& (sourceStream[encodedIndex] != ETX_CHAR)
&& (sourceStream[encodedIndex] != STX_CHAR)) {
if (sourceStream[encodedIndex] == DLE_CHAR) {
nextByte = sourceStream[encodedIndex + 1];
// The next byte is a DLE character that was escaped by another
// DLE character, so we can write it to the destination stream.
if (nextByte == DLE_CHAR) {
destStream[decodedIndex] = nextByte;
}
else {
/* The next byte is a STX, DTX or 0x0D character which
* was escaped by a DLE character. The actual byte was
* also encoded by adding + 0x40 to prevent having control chars,
* in the stream at all, so we convert it back. */
if (nextByte == 0x42 or nextByte == 0x43 or nextByte == 0x4D) {
destStream[decodedIndex] = nextByte - 0x40;
}
else {
return DECODING_ERROR;
}
}
++encodedIndex;
}
else {
destStream[decodedIndex] = sourceStream[encodedIndex];
}
++encodedIndex;
++decodedIndex;
}
if (sourceStream[encodedIndex] != ETX_CHAR) {
*readLen = ++encodedIndex;
return DECODING_ERROR;
}
else {
*readLen = ++encodedIndex;
*decodedLen = decodedIndex;
return RETURN_OK;
}
}